The U.S. Air Force spends millions of dollars each year inspecting aircraft for corrosion. Commercial aircraft owners and other governments also spend similar or greater amounts for this purpose. A significant part of this expense arises from the need to strip paint from the surface of an aircraft to detect corrosion according to present practices. This stripping is necessary because it is difficult to detect corrosion under paint by visual inspection until the paint blisters and significant damage to the painted aircraft has occurred. While inspection for corrosion remains as important as ever in order to prevent costly aircraft damage and even airframe failures, the environmental impact of the chemicals used for stripping purposes has made stripping practices even less desirable and prompted the investigation of means to detect corrosion without paint removal.
The patent art indicates aircraft manufacturers and others have also become concerned by the need to inspect aircraft for corrosion and by the related practice of stripping an aircraft in order to change its appearance or visual signature. The U.S. Pat. No. 4,647,220 of M.J. Adams et al., for example, discloses a pulsed energy and electronic scanning inclusive method of detecting corrosion located below an aircraft coating in which the corrosion detection is based on detection of surface temperature differentials resulting from exposing the aircraft to pulses of infrared energy. Notably, however the Adams et al. patent does not enjoy the advantages of detecting a phase angle lag between a pulsating energy waveform and the aircraft surface temperature undulations produced by that waveform and is thereby more sensitive to testing variations than is the system of the present invention. The somewhat related practice of stripping an aircraft in order to change its appearance or visual signature is disclosed, for example, in the U.S. Pat. Nos. 4,858,264 and 4,836,858 of T.J. Reinhart, which are assigned to the same assignee as the present patent. The patents and other documents referenced in each of the U.S. Patents identified here may also be of background interest with respect to the present invention: each of these patents Is hereby incorporated by reference herein.